Techniques in which a biphasic system for which a separated state is thermodynamically stable, such as that composed of a water phase and an organic phase, is emulsified to obtain a semi-stable emulsion have been conventionally known. For general emulsification methods, as described in "Science of Emulsions" (Asakura-shoten, 1971), methods using a mixer, a colloid mill, a homogenizer, etc. and a method of dispersion with a sound wave are known.
These general methods mentioned above have such a disadvantage that diameters of dispersed phase particles in a continuous phase distribute over a wide range.
Then, a method using filtration through a membrane comprising polycarbonate (Biochemica et Biophysica Acta, 557 (1979), North-Holland Biochemical Press); a method using repeated filtrations through a PTFE (polytetrafluoroethylene) membrane (Proceedings of the 26th Autumn Meeting of the Society of Chemical Engineers, Japan, 1993); and a method of manufacturing homogenous emulsions by sending a dispersed phase into a continuous phase through a porous glass membrane with uniform pores (Japanese Patent Application Laid-Open No. 2-95433) have been proposed.
As a method of producing emulsions using a nozzle or a porous plate, a laminar-flow dripping method (KAGAKU K OGAKU vol. 21, No. 4, 1957) is also known.
Among the conventional modified methods mentioned above, the method using filtration through a membrane comprising polycarbonate and the method using repeated filtrations through a PTFE membrane cannot manufacture emulsions of particles larger than membrane pores theoretically or cannot separate particles smaller than membrane pores. These methods are therefore especially unsuitable for producing emulsions of large particles.
In the method using a porous glass membrane with uniform pores, as shown in a graph in FIG. 11, when an average diameter of membrane pores is small, particle diameters distribute in a narrow range and thus homogenous emulsions can be obtained. When an average diameter of membrane pores is increased, however, particle diameters distribute over a wide range so that homogenous emulsions cannot be obtained.
In addition, in the laminar-flow dripping method, particle sizes become 1,000 .mu. or more and distribute over a wide range so that homogenous emulsions cannot be obtained.